An α-olefin oligomer obtained by polymerizing an α-olefin having 6 to 20 carbon atoms is produced mainly as raw material oil for synthetic lubricant oil such as engine oil. In particular, synthetic lubricant oil composed mainly of an α-olefin oligomer of 1-decene and an α-olefin oligomer of 1-octene and 1-dodecene of such α-olefin oligomers is significantly useful as raw material oil for industrial gear oil and high-performance engine oil. Accordingly, the demand for such synthetic lubricant oil has been increasing.
The above-mentioned α-olefin oligomer can be produced by a method in which monomers are polymerized in the presence of a catalyst, the catalyst is deactivated and the resulting mixture of α-olefin oligomers differing in polymerization degree (trimer, tetramer and pentamer, for example) are fractionated, or by other methods.
The catalyst used for the production of an α-olefin oligomer is normally an organic metal compound. Mixing of metal components contained in a catalyst in an α-olefin oligomer deteriorates the performance of synthetic lubricant oil, and hence, prevention of mixing in of metal components is necessary in order to keep the oil quality.
Patent Document 1 discloses that, for the method for producing an olefin-based polymer in which an organic aluminum compound is used as a catalyst, metal residues of catalyst components can be decreased by adding an oxygen-containing compound having an active proton to a reaction solution after the polymerization and adding water and an alkaline substance such that the pH of an aqueous phase becomes 9 to 13.
Patent Document 2 discloses that an aqueous base phase is mixed with a reaction solution to deactivate a catalyst, the aqueous base phase contains deactivated catalyst components and an organic phase containing an oligomer is separated from the aqueous base phase. Further, Patent Document 2 discloses that, in the method for producing a poly α-olefin, catalyst residues are adsorbed after polymerization by contacting a solid adsorbent, followed by filtration, and alumna, acid clay, Celite or the like are used as the solid adsorbent.